Manual booster and automatic pilot servo system



May 11, 1954 P. J. CHENERY ET AL MANUAL BOOSTER AND AUTOMATIC PILOT SERVO SYSTEM Filed Dec. 13, 1951 Patentecl May 11, 1954 MANUAL BOOSTER AND AUTOMATIC l PILOT SERVO SYSTEM Peter J. Chenery, Pelham,

and John C. Newton,

Roslyn Heights, N. Y., assignors to The Sperry Corporation, a corporati on of Delaware Application December 13, 1951, Serial No. 261,502 18 Claims. (Cl. 244-76) This invention relates to a manual booster and automatic pilot servo system for dirigible craft. In the practical embodiment of the invention ishown in the drawing, the improved system is illustratively depicted as employed on an aircraft in which the power motor of the system is operatively connected to a steering member such as the craft's elevator surfaces.

One of the objects of the present invention is to provide asystem `utilizing manual and automatic control for the steering member of a idirigible craft in which a single power motor performs a booster function for each of the controls and in which the effective automatic control can be overridden by the manual control.

, Another object is to provide a system of the described character in which the manual and automatic control function are also usable alternatively. i

One of the features of the invention resides in the combination in the system of an additional servomotor for the automatic pilot thereof.

A further feature of the invention is provided by a two position settable device in the mechanical repeat back connection for the power motor. Switching means settable in automatic pilot or manual control positions is eifective `to determine the corresponding setting of the settable device. In the automatic pilot position, the device is set to permit limited yielding in the repeat back connection to the manual controller to facilitate manual override of the automatic pilot controller. In the manual position, the device functions to provide initial freedom of the manually operable control member without restraint.

Other objects, features and structural details of the invention will be apparent from the following description when read in relation to the accompanying drawing in which:

Fig. l is a diagrammatic view showing the mechanical and electrical components forming the improved system and the circuitry for the related electrical parts, and

Fig. 2 is a detail view of the settable device in the repeat back connection showing the same positioned to accommodate manual control of the system.

With reference to the drawing, the relatively iixed locations of the frame of a dirigible craft employing the improved servo system are indicated at I 0. The element corresponding to the steering member for the craft is designated at II. Member Il may be the elevator control surfaces for an aircraft that are movable to change thegattitude of the craft labout its pitch axis.

As shown, member is pivotally mounted on a portion of the xed frame I 0 of the craft, the same being operatively connected to the power motor I2 of the system by way of a rod I3 extending from one end of the housing I4 of the motor I2, a rocker arm I5 connected at one end to the rod I3 and pivotally mounted to the xed frame of the craft as indicated at I6, and an interconnecting link between the member I I and rocker I5. The power motor I2 shown is an hydraulic servo of a conventional type in which the cylindrical housing is mounted to move relative to a fixed piston I8 whose rod I9 extends exteriorly of the housing I4 and is pvotally anchored to the frame I0 of the craft as designated at 20.

The element corresponding to the means for operating the power motor I2 may be provided, as shown by a conventional spool type balanced valve designated at 2| that is supplied with fluid at a constant high pressure from a suitable fluid pump source (not shown) by way of 4intake pipe 22. Valve 2| is of such character that the power motor I2 operates at a Velocity that is substantially proportional to the displacement of the valve spool 23 from its neutral position. in which the lands 24, 25 of the valve cover the ports 26,

21, controlling passage of the uid to and eX- haust of the uid from the power motor I2. The exhaust pipes for the valve 2| are indicated at 28 and 29, the same returning the duid from the motor l2 to a suitable sump (not shown). The lhousing for the valve 2| as shown may be formed as a part of the housing of the power motor I2 so that these elements of the system move as a unit. With movement of valve spool 23 to the right as viewed in the drawing, high pressure iluid from pipe 22 is admitted to the portion of the cylindrical housing I4 of the motor I2 to the right of piston I8 by way of port 2l. At the same time, the fluid to the left of the piston I8 is connected to the sump by way of port 26 and pipe 28. 'I'he power motor housing I4 together with the control valve housing is confsequently urged to the right to follow the movement of the valve spool 23 until port 21 is covered by land 25 and port 2B is covered by land 24. The power motor, when so operated, urges the rocker I5 in a clockwise direction about pivot IS and through link |'I provides down elevator. A reversal of the initial movement of the valve spool 23 from its neutral position will result in a reversal in the described operation and cause the power motor to provide up elevator.

The system further includes a manually operablecontrol member indicated in the present vrvomotor.

:tion or link "45.

v"between such -as shaft- I. 'from synchro 5l] is fed back'to the amplifier and `detector element 43 in opposition tothe input Asignal to element 43from the control gyro 39.

. from which the member may be rocked about its axis 3| by manual eiort. As shown herein, an input connection'is .provided between the lever and the balanced valve 2i providing the means for operating the power motor l2. A further input connection is provided for the valve 2| from the servomotor of the automatic vpilotmeansas hereinafter described. The valve2 I "nasa `single input obtained in the arrangement shown 'from a linkage mechanism in the `.form of adiierential rocker 33 pivotally connectedat Edito .the extending end of the valve spool E3 f thep'ower motor control means. Theeinput .connection ,bee

` tween element 30 and the linkage mechanism includes link 35, rockerV 35 pivotally mountedrat 31 to the fixed frame le of the craft, and link 38 connected toA one of the endsof the rocker 33.

The improved system further includes `automatic pilot means for controlling thesteering member' I l. As herein diagrammatically shown, such means may include-a :controlgyrondesignated at 39. This may be, for example, a ccnventional gyro vertical with a'known `A. C. pickoff (not shown) such as a-selsyn type .transmitter providing a signal inY accordance with departure .of the craft from a reference position about its i athwartships or pitch axis. The signal` from the Ygyro control element 331s. fed by way of lead/Ml, 'through an off-on switch 41, and lead '42 to 'a ysuitable Yamplifier and detector element designated diagrammatically at 43. The D. '.C. out- 'put kof 4the-element '43 lisrfed by way Vof lead 44 to .the windings'of the automatic pilotservomotor indicated at M15. As hereinshown, the vservomotor I5 is a compound electricalD. C. torque motor ofthechara'cter described and shownfin .U. S. Letters `Patent-No.2,426,508;issuedSeptem- 'ber 2, 1947, to P..Halpert .et al. The pole providing magnetic i stator element lof `motor "45 is xedly mountedon a plate .connected to'the `balanced valve `2| and 'powerfmotoril assembly.

".lhemovable element or armaturell of the servvomotor 45 is rockably mounted at "41 on .plate 46 ."for limited movement inthe gaps'between the pole faces of the .stator element of the'sermovement of `the armature `116 is operatively vconnected to'rocker 331through an input'conne'c- As Shown, link w49 is vconnected .to rocker viabove its pivot.34 andlink 38 isconnectedk to rocker'33 below its pivot 34 so ythat the valve spool 23 is operatedby both'th'e'manually foperable control "means v3U and the automatic pilotservomotor 4115. vTheautomatic pilot means may `also include a repeatbackrmeans providing sa signal in accordance'with` the .displacement of n,

.theisteering member Ilv or elevator from'ap'null condition. Such means 'may' beprovided as .hereinsshown by 'a`repeatback'synchro 'or electrical selsyn .transmitter 5D Whose stator is fixed to the craft and whose rotor is adapted to move with movement ofmember Il by aroonnectionthere- The repeatback signal This'feedback may be accomplished by way of lead 52, off-0n switchV sa'fand leader..

Spring 'centering means vare provided vfor the Vmanually operable lcontrol Amember '130 to :maintain y'the "feel of the foperators control on *the Lever arm'48 adapted'to move with;`

lber 3l, 1940, to B. G. Carlson. Y The trim adjust- ,ment required Vfor the condition of operation of fthe-craftlmay 'be made by the pilot through a means'including a suitable shaft and gear and :rack -arrangement., Ias` shown.

"Theimproved Ysystem further includes means A'for -centeringthe input connection between the .automatic .pilot .servomotor `45 and the power motor operating means provided by valve 2l. As hereinnshown, such centering means may include a centering piece 59 having a tapered surface Ssituated at the end of a lever-arm i641 pivoted at V6I `onzan extension of the plate 115 and ,-a tcooperating lug 62 fixedtolinkl) having awedge Vs'l1ape'd-,tip. Spring 53 is effective when the fsystem is set for manual operation -to force the :centeringpiece 59 on the tapered end ofthe lug to urge link 4S in the proper direction to 'center the link 49, i. e., the position the link would-fassume withfanull input to the servomotor i5 of the automaticrpilot means. The lsystem also includes a switching device or means that is `settable in automatic pilot or manual position. 'Such a means is rindicated by the.onoi switch 54. Means controlled by .theswitch' for. operatingrthe centering means include "a Vsolenoid 65 energized .from Ybattery't whenY switch Slis closed through a `closed circuit including.` lead 61 ,1 switched, .lead 68, Jbattery 66, Vlead 69 .and the ywindingiof .the solenoid. The spring restrained armature 1.0 then effective .to rockthe lever T1l yto disengage lthe cam 12-on lever V1| .and cam 1S-:on arm E0. The armBEl is'fthen vurged by 'spring 53 torengaging relation with lug'ztocenter the link 49. The means for disabling the centering means controlled bythe switching device or means 54 includes; spring restrained larmature 18, .lever 1l and fthecooperatingcams 12 and v13 which are .effective to urge'arm Guin a counterclockwise :direction to theposition in'whichiit is shownin Fig. .1. `In vthe drawing, the 4automatic and manual positions of .the two position switches I64, 53 and `4| l-are vrespectively indicated ,bythe designations (A) 'and (M). .#As'shown,the.arms tof rthe respectiveswitches 54, 53 and 4l are interconnected -by' suitable mechanical connections 14 so thatthefsame may beset by Va common knob designatedat 15in relation to switch 53.

'The improved servo system 'also includes a repeatback connectionlbetween the power .motor Vl2 and .theinput-connectionor link .38 operated iby th'eimanual controlrmember 30. The .repeatback-:connection shown in Fig. 1 is providedby rocker arm I5 connectedzto. the power 'motor 12, a "settable device 4or means designated at 1,6

situatedatthepppositeend of the rocker l5, and l.tan1extensionof link 35 having a pin 11 vlocated 'spective #ends ofthe vtong elements and the'pin `1-1 fas-*shownnfFig 1. "Inautomatic pilotpo'sition, the spring 83 also urges the upper legs of the tong elements 80, 8l to a relatively closed condition with respect to a cam member 84 situated therebetween and pivotally mounted on rocker I5. Cam member 84 is maintained in the position shown in Fig. '1 representing the automatic pilot position of the servo system by a lever 85 connected to the cam member and a spring 86 anchored to the rocker I5. The means controlled by the switching device or means 64 for changing the setting of the settable device 16 to provide manual operation of the servo system in exclusion of the automatic pilot includes solenoid 81 whose winding is parallel connected to leads 61, 69v by way of leads 89, 88. The armature 90 of solenoid 81 is connected to lever 35, the same moving lever 85 and cam member 84 to the positions shown in Fig. 2 when switch 64 is closed or set in the (M) position. This spreads the up- .per legs of the tong elements 80 and 8|. The spring 83 is stretched in this operation to permit initial clearance or freedom between the pin 11 and the lower ends of the tong elements. The automatic pilot position defined by the device 15 provides yielding restraint in the repeatback connection to member 30 by which it is possible to effect manual operation of the servo system by overriding the automatic pilot means without disabling the same.

The second or manual setting of device 16 separates rocker I and link 35 to allow suil'cient `play in the manual control member 38 for the pilot to start the booster system in operation. In this second or manual position, the device 16 provides initial freedom without restraint in the repeatback connection.

In manual operation of the booster servo system, knob is set in the (M) position, the switch 53 is opened so that the automatic pilot repeatback means is rendered ineffective, switch 4I is opened so that there is no signal input to ampliiier-detector unit 43 from the control gyro 39, and switch 64 is closed so that link 49 is held in centered position and device 16 is set in manual position with its parts in the condition shown in Fig. 2. Counterclockwise movement oi member results in a clockwise movement of rocker 3'6 which reects in motion of the spool 23 of the balanced valve ZI to the right as viewed in Fig. 1 and corresponding operation of the power motor to provide down elevator.

In automatic pilot operation, the parts are positioned as shown in Fig. 1, the automatic pilot functioning through servcmotor 45 and input connection 49 to operate the power motor I2 so that the elevator is positioned with zero input to the amplifier and detector unit 43. Through the mechanical repeatback connection, the control member 30 is caused to follow the operations of the servo system effected through the automatic pilot means. The yielding device 16 in the mechanical repeatback connection to the valve 2| permits the automatic pilot means to be` overriden by the manual control member 36. In such operation, knob 15 is left in the (A) position and the booster servo system is operated as described for manual operation through movement of the member 30 and its positioning of valve spool 23 of valve 2l through link 35, rocker 3E, link 38 and rocker 33 as permitted by the yielding connection 16. When the system is operating under control of the automatic pilot means, the ratios of the selected spring restraints provided by springs 83 of device 16 and springs 56 and 51 of the articialfeel device determines the desired range of operation of the steering member l I responsive to an operating signal from the control gyro 39. For a particular trim adiustment of the spring feel device, the linear springs 56-51 simulate the air load on member I I for the pilot within the range of operation of the control surface member. The movement of stick or member 38 corresponds to the movement of surface II. With no manual effort on the feel device and the system operating under automatic pilot control, the force necessary to deflect springs 56, 51 is supplied by spring 83. When the spring force supplied by spring 63 is equal and opposite the spring force 56, 51, then any further effort of the automatic pilot means to deilect control member o1' surface II further will result in an opening of the tongs 80, `=BI of the device 16 to limit the extent of movement of surface II as desired. This is accomplished by nuliication of the automatic pilot means control input signal through link 49 effective through the mechanical repeatback connection :including link 35, rocker 35 and link 38. Assuming the spring 83 as capable of exerting a maximum chosen force and springs 56, 51 as capable of resisting more than the maximum force of the spring 83, spring 83 is eiective to transmit the motion of member II through link I1, rocker I5, device 16, and link 35 to move lever 3U and rocker 36 until the force of the spring 83 is balanced by the restraining force of springs 56, 51. At such point, the tongs 8D, 8l of device 16 open. The motion communicated through rocker 35 and link 38 to rocker 33 then ceases. In normal automatic pilot operation, the lower pivot point of the rocker 33 follows the motion of the power motor housing I4. When such following motion ceases by reason of the opening of the tongs of settable device 16, the valve spool 23 is eiective to limit the operation of the power motor I2. Device 16 provides a spring controlled disengageable connection in the repeatback between the power motor I2 and the input connection to Valve 2I from manual member 30. The arrangement and relative restraints provided by the described spring centering means and the spring controlled disengageable connection is such that the disengageable connection becomes disengaged to effect the desired limitation of operation of the power motor.

The system is effective to enable the steering member or surface I I to be operated manually when the power motor I 2 is disabled. In such an event the switch knob 15 is set in the (M) position and the safety valve 9| connecting the opposite sides of the power motor cylinder is: opened. Direct operation of the surface II from member 30 is obtained by way of link 35, pin 11, device 16, rocker I5 and link I1.

Since many changes could be made in the above construction and many apparently widely diierent embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim:

1. In a dirigible craft having a steering member, a manual booster and automatic pilot servo system including in combination, a power motor connected to operate the steering member, a manually operable control member, automatic pilot means including a servomotor, means for operating said power motor, a first input connection;` between saidv manually operable controll member andsaid power motor operating means, a second inputV connection between the servomotor of said automatic pilot means and said power motor operating means, a switching device settableA in automatic pilot or manual position, means for centering the second input connection tosaid power motor operating means, and means controlled by said switching device when set in manual position for operating said centering means.

2. In a dirigible crait having a steering member, avmanual booster'and automatic pilot servo system. including in combination, a power motor connected to operate the steering member, a manually operableV control member, automatic pilotmeans including a servomotor, means for operating said power motor, a first input connection between said manually operable control member and said power motor operating means, a second input connection between the servomotor` of said automatic pilot means and said power motor operating means, a switching device settab'le in automatic pilot or manual position, means for centering'the second input connection to said power motor operating means, and means for'disabling said centering means controlled by saidr switching' device when set in automatic pilot position.

3, in a dirigible crait having a steering member, a manual booster and automatic pilot servo system including in combination, a power motor connected to operate the steering member, a manually operablecontrol member, automatic pilot means `including aservomotor, means for operating said power motor, a first input connection between said' manually operable control member'and Said power motor operating means, a second input connection between the servomotcr of said automatic pilot means and said power motor operating means, a repeatbaclV connectionl between the power motor and the manually operable control member, and a yielding device having limits in said repeatback connectionV facilitating manual override of said automat-ic pilot means.

4. Ina dirigible crait having a steering member, a manual booster and automatic pilot servo system including in combination, a power motor connected to operate the steering member, a manually operable control member, automatic pilot means including a servomotor, means for operating said power motor, a iirst input connection between said manually operable control member and said power motor operating means, a second input connection between the servomotor of said automatic pilot means and said power motor operating means, a repeatback. connection between the power motor and the manually operable control member, a settable device with limitsinv the repeatback connection having an override yielding position and a second position providing initial freedom without restraint, switching means settable in automatic pilot or manual position, and means controlled by said switching means for setting said device in the override position when the switching means is set in the automatic pilot position.

5. In a dirigible craft having a steering member, a manual booster and automatic pilot servo system including in combination, a power motorconnected to operate the steering member, a-V manually operable control member, automatic pilot. means including a servomotor, means for voperating said power. motor, a rst input connection between; said manually operablef cena trol member and said power motor' operating; means, a second input connection between the` servomotor of 'said` automatic pilot means and said 'power motor operating means, a 'repeat-Y back. cormection between the power motor and the manually operable control member, a set-4 table device with limits inthe repeatback connection having an override yielding position and a second position providing initial ireedomwithout restrain, switching means settable in automatic pilot or manual position, and means controlled by said switching means for setting'said` device in the secondY position when the switch-- ing means is set in the manual position.

dln a dirigible craft having a steering meinbcr, a manual booster and automatic pilot servo` system including in combination, a power motor connected to operate the steering member, a power motor connected to operate the steering member, a manualh operable control member, automatic pilot means including a servomotor, means for operating said power, motor, a ilrst input connection between said manually operable control member and said power motor operat ing means, a second input connection betweenthe servomotor ci said automatic pilot means and said power motor operating means, a switching device settabie in automatic pilot or manual position, means for centering the second input connection, a repeatbacli connection between the power motor and the manually operable control, means, settable means with limits in the repeatbaci: connection having an override yielding position and a second position.` providing initialV freedom without restraint, and means controlled by said switching device when set in manual position for operating said centering means and for setting said settable means in said second position.

7. In a dirigible craft having a steering niember, a manual booster and automatic pilot servo system including in combination, a power motor connected to operate the steering member, a manually operable control member, automatic pilot means including a servomotor, means for operating said power motor, a iirst input connection between said manually operable control member and said power motor operating means, a second input connection between the servomotor of said automatic pilot means and said power motor operating means, a switching device settable in automatic pilot or manual position, means for centering the second input connection, a repeatback connection between the power motor and said manually operable-control means, settable means with limits in the repeatback connection having an override yielding position and a second position providing initial freedom without restraint, means for-disabling said centering means controlled by said switching device when set in automatic pilot position, and means controlled by saidswitching device when set in automatic pilot position for setting said settable means in said override yielding position.

8. In a dirigible craft having a steering member, a manual booster and automatic pilot servo system including in combination, a power motor connected to operate the steering member, aimanually operable control member, automatic pilot means including a servomotor, means for operating said power motor, a differential rocker providing an input to said power motor-'operat-` ing means, a nrst input connection to said differential rocker operatively connected to said automatic pilot servomotor, a second input connection to said differential rocker operatively connected to said manually operable control means, a switching device settable in automatic pilot or manual position, means for centering said rst input connection, and means controlled by said switching device when set in manual position for operating said centering means.

9. In a dirigible craft having a steering member, a manual booster and automatic pilot servo `system including in combination, a power motor connected to operate the steering member, a

manually operable control member, automatic pilot means including a servomotor, means for operating said power motor, a differential rocker providing an input to said power motor operating means, a first input connection to said differential rocker operatively connected to said automatic pilot servomotor, a second input connection to said differential rocker `operatively connected to said manually operable control means, a switching device settable in automatic pilot or manual position, means for centering said iirst input connection, and means for disabling said centering means controlled by said switching device when set in automatic pilot position.

10. In a dirigible craft having a steering member, a manual booster and automatic pilot servo system including in combination, a power motor connected to operate the steering member, a manually operable control member, automatic pilot means including. a servomotor, means for operating said power motor, a differential rocker providing an input to said power motor operating means, a first input connection to said differential rocker operatively connected to said automatic pilot servomotor, a second input connection to said differential rocker operatively connected to said manually operable control means, a repeatback connection between said power motor and said second input connection, and a yielding device having limits in said repeatback connection permitting manual override of said automatic pilot means.

11.A In a dirigible craft having a steering member, a manual booster and automatic pilot servo system including in combination, a power motor connected to operate the steering member, a i

manually operable control member, automatic pilot means including a servomotor, means for operating said power motor, a diierential rocker providing an input to said power motor operating means, a first input connection to said differential rocker operatively connected to said automatic pilot senvomotor, a second input connection to said differential rocker operatively connected to said manually operable control means, a repeatback connection between said power motor and said second input connection, a settable device with limits in the repeatback connection having an override yielding position and a second position providing initial freedom without restraint, switching means settable in automatic pilot or manual position, and means controlled by said switching means for setting said device in the override position when the switching means is set in the automatic pilot position.

` 12. In a dirigible craft having a steering member, a manual booster and automatic pilot servo system including in combination, a power motor connected to operate the steering member, a manually operable control member, automatic pilot means including a servomotor, means for operating said power motor, a diierential rocker providing an input to said power motor operating means, a rst input connection to said differential rocker operatively connected to said automatic pilot servomotor, a second input connection to said differential rocker operatively connected to said manually operable control means, a repeatback connection between said power motor and said second input connection, a settable device with limits in the repeatback connection having an override yielding position and a second position providing initial freedom without restraint, switching means settable in automatic pilot or manual position, and means controlled by said switching means for setting said device in the second position when the switching means is set in the manual position.

13. In a dirigible craft having a steering member, a manual booster and automatic pilot servo system including in combination, a power motor connected to operate the steering member, a manually operable control member, automatic pilot means including a servomotor, means for operating said power motor, a differential rocker providing an input to said power motor operating means, a first input connection to said differential rocker operatively connected to said automatic pilot servomotor, a second input connection to said diierential rocker operatively connected to said manually operable control means, a switching device settable in automatic pilot or manual position, means for centering said rst input connection, a repeatback connection from the power motor to said second input connection, settable means with limits in the repeatback connection having an override yielding position and a second position providing initial freedom Without restraint, and means controlled by said switching device when set in manual position for operating said centering means and for setting said settable means in said second position.

14. A system as claimed in claim 13, including spring centering means for said manually operable control means.

15. In a dirigible craft having a steering member, a manual booster and automatic pilot servo system including in combination, a power motor connected to operate the steering member, a manually operable control member, automatic pilot means including a servomotor,` means for operating said power motor, a differential rocker providing an input to said power motor operating means, a irst input connection to said differential rocker operatively connected to said automatic pilot servomotor, a second input connection to said differential rocker operatively connected to said manually operable control means, a switching device settable in automatic pilot or manual position, means for centering said first input connection, a repeatback connection between the power motor and said second input connection, settable means with limits inthe repeatback connection having an override yielding position and a second position providing initial freedom without restraint, means for disabling said centering means controlled by said switching device when set in the automatic pilot position, and means controlled by said switching device when set in automatic pilot position for setting said settable means in said override yielding position.

16. A system as claimed in claim 15, including spring centering means for said manually operable control member.

17. In a dirigible craft havinga steering member, a manual booster and automatic pilot servo system including incombination, a power motor connected to operate the steering member, a manually operable control member, automatic pilot means including a servomotor and repeatback means providing a signal in accordance with the displacement of the steering member from a null condition, means for operating said power motor, a rst input connection between said manually operable control vmember and said power motor operating means, a second input connection between theV servomo-tor of said automatic pilot means and said power motor operating means, a switching device settable in automatic pilot or manual position, means for centering said second input connection, and means controlled by said switching device when set in manual position for operating said centering means and for rendering said automatic pilot repeatback means ineffective.

18. In a dirigible craft having a steering member, a manual booster and automatic pilot servo system including in combination, a power motor connected to operate the steering member, a manually operable control member, automatic pilot means including a servomotor, means for operating said power motor, a rst input connection between said manually operable control member and said power motor operating means,

a second input connection-between the servomotor of said automatic pilot means and said power motor operating means, spring centering means for said manually operable control member, means for limiting the operation ci 'said power motor under control of said automatic pilot means including repeatbac-k means between said power motor and said rst input connection vhaving a springrcontrolled disengageable connection therein, the arrangement and relative restraints provided by said centering means and disengageable connection being such that the disengageable connection becomes disengaged to eiect the desired limitation of operationof the power motor.

References Cited in thefile of this patent UNITED STATES PATENTS w A van?" 

